Photoelectro-chemical cell for conversion of solar energy to electricity and method of its manufacture

ABSTRACT

A double photoelectrochemical cell for converting solar energy directly to electricity. The device in one embodiment has two semiconductors which are separated from each other by a polymer electrolyte. The two semiconductors have a different band gap; for example, one is n-type CdS and the other is p-type CdTe. The polymer electrolyte, for example, is a thin film polyethylene oxide acting as a polymer matrix containing a polysulfide redox couple, for example, Na 2  S 4 . The polymer electrolyte is transparent, insulating and capable of transporting ions. At least one of the semiconductors is semi-transparent. The short wavelengths of light are absorbed by the semi-transparent wide band gap semiconductor, and the long wavelengths pass therethrough, and through the polymer film electrolyte, and are absorbed by the narrow band gap semiconductor. The output of the cell is double; one from each semiconductor, i.e. they are series-connected. Output voltage, for example, is about 0.625 volts. The theoretical efficiencies in the example are about 35%, compared with about 25% for a standard photovoltaic cell, having a single junction. Also included is a method of manufacturing such a cell.

This is a division, of application Ser. No. 208,059, filed Nov. 18,1980, now U.S. Pat. No. 4,352,868.

BACKGROUND OF THE INVENTION

1. Field of the Invention:

The invention relates generally to photovotaic cells and methods oftheir manufacture, and more particularly to such cells employing thisfilm polymer electrolytes and methods of their manufacture.

2. Description Of The Prior Art:

Photovoltage or the photovoltaic effect may be defined as the conversionof light or electromagnetic photons to electrical energy by a material.Becquerel in 1839 was the first to discover that a photvoltage developedwhen light was shining on an electrode in an electrolyte solution.Nearly half a centry elapsed before this effect was observed in a solid,namely in selenium. Again, many years passed before successful devicessuch as the photoelectric exposure meter, were developed. Radiation isabsorbed in the neighborhood of a potential barrier, usually a pnjunction or a metal-semiconductor contact or junction, giving rise toseparate electron hold pairs which create a potential.

Photovoltaic cells have found numerouse applications in electronics andaerospace, notably in satellites for instrument power, and poweringcommunications apparatus in remote locations.

Intensive research in underway in the last decade to improve theproduction of these cells, e.g., (1) increasing the practical efficiencyin order to approach the theoretical efficiency, (2) decreasingproduction costs, and (3) to find new materials and combinations.

Intensive interest in alternative energy sources and particularly insolar energy has increased because of political and economic impetus.Traditional sources of inexpensive energy are rapidly disappearing.Political instability, price/supply fixing by certain governments, andenvironmental concerns, dictate the search for new energy sources. Thusthe present intense interest in solar energy. Each country has its ownsunlight supply, and the United States has an ample supply.Ecologically, solar cells are a non-polluting clean source of energy.Solar energy in our forseeable future for many generations is limitlessand non-depletable. One application of solar energy to which the presentinvention is directed is the direct conversion of electromagneticradiation, particularly sunlight, to electricity.

Two of the classical goals of any photovoltaic cell are efficiency, andhigher output voltage. Most prior art cells have a theoreticalefficiency of about 25%. The cells of the present application approach35%. The prior art voltage ranges from 0.2 to 0.5 volts per cell; theinventor's cells are approximately 0.625 volts.

Further, some prior art cells require that they be oriented so that theincident light is perpendicular to the face of the cell. In the presentinvention, while this is desirable, it is not essential, and they mayoperate at an angle from the perpendicular.

The present invention offers the possiblity of ease of manufacture,attendant low cost, and manufacturing large surface areas with goodquality and at a low cost.

The present invention is corrosion free. A reduction oxidation couple inwater has a photocorrosion resulting from an interaction between thewater and semiconductors. The present invention by using a polymermatrix avoids photocorrosion and the attendant problems.

An object of the present invention is to provide a novel, doublephotoelectric cell for conversion of solar energy to electricity.

Another object of the invention is to provide a method for themanufacture of double photoelectrochemical cells. A further object ofthe invention is to provide a half-double photochemical cell for theconversion of solar energy to electricity using a thin film polymerelectrolyte.

A further object of the invention is to provide a new family ofphotoelectrochemcial cells having a theoretical higher output efficiencyand output voltage than is available from single cells.

Another object of the invention is to provide cells which are easy tomanufacture and are stable in operation.

These and other objects and features of the invention will be more fullyunderstood from the description of the embodiments which follow, but itshould be understood that the invention is not limited to theseembodiments and may find application as would be obvious to a manskilled in the art following the teachings of this application.

SUMMARY OF THE INVENTION

According to one aspect of the invention, there is provided a device forconverting electromagnetic radiation, preferably sunlight, directly toelectricity. It includes a first semiconductor having a first band gapand a second semiconductor adjacent thereto having a different band gap.A thin dry solid film polymer electrolyte separates the first and secondconductors. The polymer electrolyte is a transparent, insulating polymercapable of transporting electric charges between the two semiconductorsvia the transport of ions and/or electrons due to electroactivemolecules or ions dissolved in the polymer matrix or attached to thepolymer backbone. One of the semiconductors is partially transparent andabsorbs part of spectrum of the electromagnetic radiation or light; andthe other absorbs other parts of the spectrum. Electrodes are connectedto the semiconductors for collecting the output voltages from thesemiconductors.

According to further aspects of the invention, there is provided adevice having a transparent counter electrode of a thin film or grid, athin film polymer electrolyte adjacent thereto; a semiconductor adjacentto said polymer electrolyte film and an electrode adjacent to thesemiconductor.

According to another aspect of the invention, there is provided a methodof manufacturing such cells.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a double photoelectrochemical cellaccording to the invention.

FIGS. 2 and 3 are band diagrams of the semiconductor elements of FIG. 1when the cell is without incident radiation, and when it receivesradiation respectively.

FIG. 4 is a schematic diagram of a second embodiment of the inventionshowing a half cell.

FIGS. 5 and 6 are band diagrams of metal polymer semiconductor devicesof FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1 there is schematically shown the photoelectrical cell havingtwo semiconductors, 1 and 2, separated by a polymer electrolyte 3.Incoming electromagnetic radiation, for example, sunlight, is shown byan arrow 4. Electrodes 5 and 6 are connected to the semiconductors 1 and2, respectively. The electrodes are connected by leads to a load shownhere as a meter 7.

Semiconductor 1 is a this film of cadmium sulfide, CdS, doped n-type andis a thin film, approximately 1 micrometer thick. Actually, the CdS inthe undoped form is supposed to be n-type due to vacanices, according tomy sources. As shown in FIGS. 2 and 3, CdS n-type has a wide band gap.Semiconductor 2 is a thin film of cadmium telluride, CdTe, and is dopedwith a p-type dopant. p-type CdTe has a narrower band gap than that ofn-type CdS, as shown in FIGS. 2 and 3. The two semiconductors 1 and 2face each other, and are in contact with and separated by the thin filmpolymer electrolyte 3.

The polymer electrolyte is an electron and/or ion-exchange polymer, forexample, a polymer matrix containing a redox reduction-oxidation couple.The polymer matrix is a polyalkene oxide. Polyethylene oxide has beentried and operated satisfactorily. Polyethylene glycol, polypropyleneoxide, or polypropylene glycol should also form suitable polymermatrixes. The redox couple is a polysulfide, e.g. a polysulfide whichhas been used by the inventors is Na₂ S₄. Other couples may be used. Thepolymer electrolyte film was made on the CdS 1 by evaporation from amethanol solution, i.e. polyethylene oxide with a methanol solvent.Contact with the CdTe semiconductor 2 was by contact and heating undervacuum with a pressure of 1 kg/cm².

Electrical leads shown schematically as 5 and 6 in FIG. 1 are connectedto the walls of the semiconductor films 1 and 2. The leads may be anyconvenient or conventional transparent electrical lead. If the incidentlight 4 falls on the semiconductor 1, then lead 5 is a grid, ortransparent electrode, at least for those portions of the spectrum whichare absorbed by the semiconductors 1 and 2. Lead 6 may also betransparent to permit light to enter from both sides of the cell.Alternatively, electrode 6 may be reflective itself or have reflectivematerial on the far side from the light 4, in which case anynon-absorbed radiation would be reflected and further absorbed.Electrodes 5 and 6 are connected to leads shown schematically and whichin turn are connected to a load, shown here as volt meter 7. A substrate(not shown) is provided as well as suitable mechanical protection forthe electrodes and semiconductors. The substrate and protective filmmust be transparent over that portion of the cell through which lightpasses. Glass is the usual substrate, although plastic substrate inencapsulation may also be used. The electrode facing the incident lightmay have an antireflection coating.

FIGS. 2 and 3 are band diagrams of the device of FIG. 1. FIG. 2 showsthe device in the dark, and FIG. 3 the band diagram under illumination.The band gap of n-type CdS is typically 2.4 eV. The band gap of p-typeCdTe is typically 1.45 eV. In the dark, the Fermi level ^(E) F is thesame. Under illumination as shown in FIG. 3 with the incidentillumination shown schematically by the wavy line with the legend 14,the Fermi levels shift, and there is a net potential across thesemiconductors of V=^(E) F.sup.(n) -^(E) F.sup.(p). The twosemiconductors have different band gaps. There is thus a multi-colorcell which divides the solar spectrum into two parts, with the shortwavelengths absorbed by the wide band gap semiconductor 1, and the longwavelengths absorbed by the narrow band gap semiconductor 2. The polymerelectrolyte 3 permits the flow of charge there-across, and the twojunctions 1-2 and 2-3 are in series. The maximum theoretical efficiencyof the cell is about 35%. This can be compared with the best singlejunction photovoltaic cell commonly used having a theoretical efficiencyof 25%, that being for gallium arsenide. The theoretical efficiency of acadmium telluride cell by itself is 25%; and a cadmium sulfide cell byitself is 16%.

The open circuit voltage measured by volt meter 7 reads 0.625 voltsusing 100 milliwatt/cm² Xenon light.

Turning now to FIG. 4, there is shown an electrolytic Schottky barrierdevice or "half-cell" of the invention. In this embodiment there is asingle junction. The embodiment of FIG. 4 is similar to the one of FIG.1 except that one of the semiconductors is replaced by a metal orcounter electrode 21. Electrode 21 is a thin metal film or grid which issemi-transparent or other transparent counter electrode, e.g. tin-oxideor indium-tin-oxide, and preferably completely transparent to thatspectrum which is to be absorbed. A semiconductor layer 22 may be eitherp-type or n-type. A polymer electrolyte 23 separates the transparentcounter electrode 21 and the semiconductor 22. A transparent cover, forexample, glass 25 or an antireflection coating is on an outside face ofthe metal electrode 21. A conducting base electrode 26 is on an outsideface of the semiconductor 22. Light 24 passes through the transparentcover 25, transparent electrode 21, and electrolyte 23 and is absorbedby the semiconductor 22. The device of FIG. 4 is an electrolyticSchottky barrier cell and may have a higher open circuit voltage thanthat of the solid state junction cells.

The band diagram of the device of FIG. 4 is shown in FIGS. 5 and 6 withn- and p-type semiconductors, respectively; and with light impingingupon the semiconductors. The flow and direction of holes, h⁺, andelectrons, e⁻, are shown in the semiconductor region. The voltage acrossthe electrodes 21, 26 at open circuit is V=-E_(F) ^(semi) E_(F) ^(metal)for the n-type semiconductor, and V=E_(F) ^(metal) -E_(F) ^(semi) forthe p-type.

Thus, there has been shown and described a novel photovoltaic cell usinga polymer electrolyte. Polymer electrolytes are a new concept inphotoelectrochemical cells for the conversion of solar energy toelectricity. It is envisioned that this is a basic invention of suchdevices and the invention should not be narrowly interpreted during thelife of any patent. Those following the teaching of this applicationwill no doubt be led to other and additional polymer electrolytes andother semiconductors than those specifically described herein (which arethe ones that have been employed by the inventor in his research todate).

We claim:
 1. A photoelectric device comprising: a transparent counterelectrode; a thin dry solid film polymer electrolyte adjacent theretoand electrically interacting therewith; a semiconductor adjacent to saidpolymer electrolyte film; and a conducting base electrode electricallyinteracting with said semiconductor.
 2. A device according to claim 1wherein said polymer electrolyte is a polymer matrix containing areduction oxidation couple.
 3. A device according to claim 2 whereinsaid polymer matrix a polyalkene oxide.
 4. A device acccording to claim3 wherein the polyalkene oxide is one of the group of polyethyleneoxide, polyethylene glycol, polypropylene oxide, or polypropyleneglycol.
 5. A device according to claim 2 wherein said polymer matrix ispolyethylene oxide.
 6. A device according to claim 2, or 3, or 4, or 5wherein said reduction oxidation couple is a polysulfide.
 7. A deviceaccording to claim 6 wherein said polysulfide is Na₂ S₄. PG,15
 8. Adevice according to claim 1 or 2 wherein said semiconductor is n-typeCdS or p-type CdTe.
 9. A device according to claim 2, or 3, or 4, or 5wherein said reduction oxidation couple is a polysulfide.
 10. A deviceaccording to claim 9 wherein said polysulfide is Na₂ S₄.
 11. Aphotoelectronchemical cell for the conversion of solar energy toelectricity, comprising the combination of a dry solid polymerelectrolyte film with a semi-conductor for the absorption of incomingsunlight radiation, said semi-conductor interacting with a conductingbase electrode.
 12. A device according to claim 11 wherein said polymerelectrolyte is a polymer matrix containing a reduction oxidation couple.13. A device according to claim 12 wherein said polymer matrix is apolyalkene oxide.
 14. A device according to claim 13 wherein thepolyalkene oxide is one of the group of polyethylene oxide, polyethyleneglycol, polypropylene oxide, or polypropylene glycol.
 15. A deviceaccording to claim 12 wherein said polymer matrix is polyethylene oxide.16. A device according to claim 11 or 12 wherein said semiconductor isn-type Cds or p-type CdTe.